From @zoffixznet

https://irclog.perlgeek.de/perl6/2017-02-11#i_14084570

When duplicate replacement chars are given, the first replacement is used when no named args are present but last (or later?) replacement is used when there are any named args

16​:36 IOninja m​: say "abc".trans("abc" => "XYZ", :meows)
16​:36 camelia rakudo-moar 266f34​: OUTPUT«XYZ␤»
16​:36 IOninja m​: say "abc".trans("aba" => "XYZ", :meows)
16​:36 camelia rakudo-moar 266f34​: OUTPUT«ZYc␤»
16​:36 IOninja m​: say "abc".trans("aba" => "XYZ")
16​:36 camelia rakudo-moar 266f34​: OUTPUT«XYc␤»
16​:37 ufobat ha :)
16​:38 IOninja ufobat​: looks like indeed a bug (inconsistency) on how duplicates are processed between .trans with named args and without. It follows one codepath without any named args and if there are any, changes the replacement to list form and that apparently uses different path for dups
16​:38 ufobat should i, will you create a bug report?